At first it seems like it might be nigh impossible, but you may notice that each saved Babelian and each destroyed Nebulon is worth progressively more points as the levels continue.
By the 7th level, the Babelians are worth around 1000 points apiece. So, while the levels do get progressively harder, it gets progressively easier to score points.
Depending on how many Nebulons you shoot (and how many you let escape with a living Babelian) you should be able to score 50k points by getting mid-way through level 7.
One thing I hadn't really paid attention to was whether or not you lose points for shooting "hostage" Babelians, and whether or not it changes the number of points you lose for letting a Nebulon escape with one.